Traditionally, I've used Sea Foam in a fuel to Sea Foam mixture of 5:1 or 6:1, depending on my perception of how carboned up the engine is. This works on any gas engine - lawn mowers, power washers, generators, etc. Warm up the engine on regular fuel. Switch over to a tank with the mixture. Run engine until you are sure mixture has gotten into the entire fuel system. The engine will let you know by starting to run rough. On the 225 you will likely use at least a gallon of mixture for each run because you want it to get past the fuel filters and VST into the fuel rails. You will likely need to bump up the rpm's to at least 1,000 to keep the engine running a bit. Let it sit for an hour or so. Change back over to regular fuel and run until it stops smoking. Repeat the procedure. If a lot of smoke after the second time, do it again. The commercial fisherman and crabbers in this area swear by Sea Foam. About every convenience store on the Northern Neck of Virginia carries it.
Then, change oil and filter. The oil will likely be super dirty.
However, the next time I do this I'm likely going to change the Sea Foam mixture to the Yamaha Ring Free mixture in a 2 ounces per gallon of fuel. The Ring Free is a lot more expensive, but supposedly does a better job. Normally, I run a 12 ounce bottle of Ring Free in 120 gallons of fuel at the beginning and end of every season - that's the recommended "maintenance" mixture.
If you can't get either there in the UK, there are supposed to be equivalent mixtures out there like OMC Engine Tuner, Merc Power Tune, and Valvetech Carbon Remover. However, I have absolutely no experience with any of those.
